(1) Field of the Invention
The present invention relates to a base station and a mobile station in a cellular radio communication system, and more particularly, to a base station and a mobile station to be applied to a cellular radio communication system of OFDM (Orthogonal Frequency Division Multiplexing).
(2) Description of the Related Art
The OFDM is a communication method for multiplexing a plurality of orthogonal carriers by performing digital modulation. A transmission station converts a digital signal generated in a frequency domain into a baseband OFDM signal in a time domain by IFFT (Inverse Fast Fourier Transform), converts the baseband OFDM signal into an RF signal having a radio band frequency, and transmits the RF signal through an antenna. A reception station converts the RF signal received by an antenna into the baseband OFDM signal, and converts the baseband OFDM signal in the time domain into a signal in the frequency domain by FFT (Fast Fourier Transform) to reproduce the original digital signal.
More specifically, the transmission station converts a data bit sequence to be transmitted into a sequence of complex modulation symbols, converts the sequence of complex modulation symbols into N parallel complex symbols by serial-to-parallel conversion, converts the N parallel complex symbols into parallel signals en bloc by IDFT (Inverse Discrete Fourier Transform), converts the parallel signals into a serial signal and converts the serial signal into an RF signal.
As the third-generation CDMA (Code Division Multiple Access) mobile communication system enables communication of multimedia information through the IP (Internet Protocol) network, there is a growing need for wider-band communication service. Accordingly, in the next-generation cellular radio communication system using the OFDM, it will be general to support wideband communication such as best-effort data communication, VoIP (Voice over IP) communication, and the distribution of streaming information such as video pictures. Further, in order to deal with increase in the amount of transmission data from terminals, OFDMA (Orthogonal Frequency Division Multiple Access) using the OFDM also to uplink access is being studied.
In the OFDM cellular radio communication system, there are formed as communication channels between a base station and a mobile station, a traffic or data channel for transmitting user data, and various control channels such as an access channel for establishing an uplink toward the base station from the terminal, a control channel for transmitting control information through the uplink and a downlink toward the mobile station from the base station, and a broadcast channel for notifying channel assignment information and system information from the base station to the mobile station.
In the case where the OFDMA is applied to upward access, a control function of adjusting the uplink data transmission timing of each mobile station is required in order to receive upward data from the mobile station in synchronization with a reference clock (base clock) of the base station. Because, owing to the restriction in the configuration of the reception unit for the base station, it is difficult for the base station to perform individual reception control for each of unspecified number of mobile stations in accordance with the signal reception timing gap, but signal reception processing becomes easier when transmission signals from a plurality of mobile stations are controlled so as to synchronize with the base clock of the base station. In this case, TA (Time Alignment) control to feed back, from the base station to each mobile station, a control signal for correcting data transmission timing of the mobile station is required.
In the TA control, each mobile station transmits to the base station a synchronization signal indicating the reference timing of data transmission at the mobile station, by using a control channel dedicated to TA. Instead of using the channel dedicated to TA, the synchronization signal may be inserted in the data channel at regular intervals. The base station calculates the gap between the synchronization signal received from each mobile station and the base clock of the base station, and feeds back a transmission timing control signal to each mobile station at regular control intervals so that the gap becomes zero. Each mobile station corrects the transmission timing of data on the uplink channel according to a TA control signal which is the transmission timing control signal received from the base station.
According to the TA control disclosed in 3GPP: “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures” (TS 36.213 (v1.20), 2007.06.21), the base station adjusts the synchronization signal generation timing of the mobile station in the sample clock unit for each control period. By adjusting the synchronization signal of the mobile station little by little in this manner, it is possible to synchronize the data transmission timing of the mobile station with reception timing of the base station without interrupting communication.
Here, it is assumed that a communication environment changes with the movement of the mobile station and synchronization between the mobile station and the base station is lost. If the synchronization gap remains within the duration of the guard interval (GI) at the receiving station, deviation of the synchronization does not cause a serious problem. However, if the communication environment changes suddenly, such a large deviation that the time alignment control cannot immediately correct the synchronization may occur, that is, the data transmission timing may be out of normal TA control.
For example, in a street of office buildings, an uplink radio wave transmitted from the mobile station reflects on walls of buildings. Therefore, depending on the positional relation between the mobile station and the base station, there occur cases where a direct wave of the uplink signal reaches the base station, both the direct wave and an indirect wave reach the base station, and the direct wave is blocked because of shadowing so that only the indirect wave reaches the base station.
When the mobile station moves, for example, from a communication environment in which only the indirect wave can reach the base station to another position at which the base station can be seen without obstruction, the propagation delay time of the uplink signal changes suddenly. As a result, such a large loss of synchronization that cannot be compensated by the time alignment control occurs depending on the circumstances. A similar loss of synchronization occurs also in a downlink signal transmitted from the base station to the mobile station.